Following commercial introduction of a nozzle dam assembly described in U.S. Pat. Nos. 4,656,714 and 4,667,701, certain advances were made in its design which are now in the prior art. The common purpose of those design modifications and the present invention is to minimize personnel radiation exposure during installation by reducing installation time and to make the nozzle dam as fail-safe as possible without compromising the structural integrity of the nozzle or generator wall.
In the patented design referred to above the dam was assembled on a support frame from segments encircled by a sealing diaphragm and the assembly was then rotated into sealing position within the nozzle. One subsequent prior art modification to that patented design was to assemble the dam segments in place within the nozzle and then attach as many as 18 clamp devices around its periphery to hold it to a special clamp ring encircling the nozzle opening. The diaphragm was preaffixed to the center dam segment in that design, as it is in the present invention as well, but no support frame was utilized behind the dam and no provision was made for adjusting the operating position of the dam to the particular dimensional characteristics of the nozzle.
Another of the prior art modifications referred to above was the assembly of the dam within the nozzle opening by first installing the sealing diaphragm by itself on bosses previously welded to or screwed into the nozzle wall, then inserting the various dam segments and attaching them to the flange ring by clamps as in the first-described modification, and finally spanning the backside of the assembled dam with support beams which were releasably attached to the bosses by quick-release bayonet pin devices. While a support frame transverse to the dam segments was included in that design, as it is in the present invention, the welded or threaded bosses on the nozzle wall itself necessitated stress analyses after the bosses were put in place to insure that the nozzle wall structural integrity had not been compromised. Also there was no provision for adjusting the operating position of the dam to the particular dimensional characteristics of the nozzle.
Yet another of the prior art modificactions referred to above involved the use of a hold-down ring welded directly to the nozzle to receive the nozzle dam segments with their diaphragm seal. A number of lock pins on the dam segments were utilized to secure the dam to the hold-down ring. Again, the sealing diaphragm was preaffixed to the center dam segment, as in the present invention, but there was no provision for adjusting any of the components to the particular dimensional characteristics of the nozzle.
the average installation time for most prior art nozzle dams has remained in the order of five minutes. With the higher radiation levels which are now common there is a great need for reducing that duration of personnel radiation exposure during installation. When personnel assemble the dam components of the invention directly in the nozzle opening and secure them by a transverse support frame they can complete the installation in as little as two minutes with certainty that the dam is located at its optimum sealing position within the nozzle notwithstanding the particular nozzle profile and the particular eccentricity of the fixed flange ring relative to the nozzle opening.